The successful formation of radiation-induced crosslinking in Nafion (R) N117CS membranes was clearly demonstrated by tensile testing, a methanol uptake measurement, thermo-mechanical analysis (TMA), and fluorine-19 MAS NMR ((19)F NMR) spectroscopy. The possibility of radiation-induced crosslinking of Nafion (R) had been hardly considered with regard to the thermal stability of Nafion (R) membranes, since the scission of perfluoroalkylether pendant-chains in Nafion (R) membranes in their protonated form begins at 523 K. To improve the thermal stability of Nafion (R) membranes, Nafion (R) membranes were soaked in sodium chloride prior to irradiation. The sodium-exchange Nafion (R) membranes were irradiated with gamma rays in an argon atmosphere at temperatures ranging from room temperature to 618 K. The irradiated membranes were re-exchanged to the protonated form. The (19)F NMR results for the alcohol-swollen membranes indicate peaks which originated due to radiation-induced crosslinking. An increase in percent elongation at break, a decrease in methanol uptake, and a decrease in the softening temperature at a differential TMA peak for membranes irradiated at 598 K compared to received membranes also add support for our new finding. (C) 2010 Elsevier B.V. All rights reserved.